One well-known method for microdispensing of aqueous solutions of substances onto a carrier uses rarefaction of the medium over the surface of the solution for filling a closed calibrated space, and excess pressure for ejecting said solution from the calibrated space back into solution or onto the surface of a carrier. A device for carrying said method into effect comprises a precision pair "cylinder-piston," wherein displacement of the piston with respect to the cylinder establishes rarefaction above the piston, thereby providing for filling the below-piston space with a microvolume of solution which is ejected from said space during the return piston stroke. An increased quantity of simultaneously prepared microvolumes is provided, using this method, by an increased number of parallel channels. The method under consideration and the device carrying it into effect are capable of microdispensing of the aqueous solution of substances in the range from tens of milliliters down to one microliter with an accuracy of 0.5% to 1% (cf. Cole-Parmer.RTM. micro and macropipettors; L-07839-51.div.L-07839-76). Multichannel devices (Chempette.RTM. multichannel pipettes; L-07915-60) provide for microdispensing of quantities from tens of milliliters down to 5 .mu.l, respectively, with an accuracy of 1.5 to 2%, the maximal number of parallel channels being 12.
The method discussed before and the device for its realization cannot be used for dispensing microvolumes lesser than one microliter. Moreover, said method and device have a rather low dispensing capacity (no more than 12 parallel channels).
Another method for microdispensing of aqueous solutions of substances onto a carrier is used in BIOMEK.RTM. 1000 Automated Laboratory Workstation (BECKMAN) and intended for high-quality transfer of clones, cell lines, bacteria, aqueous DNA solutions, etc. The method of microdispensing comprises use of a rodlike transferring element. The method consists in wetting the free end surface of the transferring element with an aqueous solution of substance to form a dose of said aqueous solution on said surface, followed by transferring said dose and bringing it in contact with the surface of the carrier. The method in question is based on the action of intermolecular interaction forces at the liquid-solid interface.
Also known is a device for carrying said method into effect.
The device comprises a plate-like base, one side of which carries 96 needle-like rods which are clamped thereto at one end, the other (butt) faces of said rods being coplanar (S91-8545-AP-20; 1991 Beckman Instrument Inc.; Bulletin No. 7883).
The layout of the rods on the plate corresponds exactly to the arrangement of the wells containing the solutions, on the tray. Two standard sizes of the rods are available, differing in the diameter of the end face (0.0015" tip pins--P/N 372172; 0.060" tip pins--PIN 372173; S91-8545-AP-20; 1991 Beckman Instument Inc., Bulletin No. 7883).
Microdoses of aqueous solutions of substances are transferred from the tray wells to the carrier as follows.
The plate carrying the rods are secured in a holder joined to the moving positioner head. By moving the head one positions the plate above the tray so that one rod is located over each well, whereupon the plate is moved down along the Z-axis. As a result, the rods dip into the tray wells to get wetted with the solutions contained therein. Thereupon the plate is moved up along the Z-axis, displaced spatially, and positioned above the carrier surface. Next the plate is moved down again along the Z-axis until the rod ends get in contact with the carrier surface, whereby the microdoses of the solutions of substances are transferred from the rod ends onto the carrier. Thereupon the rods are cleaned, sterilized, and dried, after which the cycle is repeated. Thus, 1536 samples are transferred to the carrier for about 30 minutes, that is, the known method and device make it possible to increase the dispensing capacity and that of transferring aqueous solutions of substances compared with the method and device discussed before. The capabilities of said method are, however, limited to the rate of evaporation of a microdose of liquid from the transferring element, that is, the rod.
When a single microvolume is as low as tens of nanoliters or less, the solution is liable to badly evaporate and its viscosity increases to such as extent that the dose of a solution cannot be transferred to the carrier, or such a microdose evaporates completely in the course of transferring. Moreover, inasmuch as the end and side surfaces of the rods in the known device feature the same magnitude of the wetting angle, the microvolumes of solutions are formed at the vacant ends of rods as microdrops which spread over the side rod surface as well, which place limitation upon the minimal volume of the solution being transferred. For the same reasons reproducibility of the microdrop volume is affected, since it depends on the rod dipping depth (that is, the dipping depth of the rod changes in response to a drop of the solution level in the wells).